The present application claims the benefit of International Patent Application No. PCT/GB00/01761 filed on May 8, 2000, which claims priority to British Patent Application No. 9910738.5 filed on May 11, 1999.
The present invention relates to a static mixer of the type used in conduits to generate turbulent flow in fluids within the conduits.
Static mixers operate by modifying the flow of process components in a conduit. In one known static mixer, mixing elements are placed in the conduit to split the flow, rotate the flow stream, and then re-integrate the flow stream a number of times to achieve the desired mix. A different known type of static mixer achieves mixing by the use of deflector elements extending into the conduit to create turbulence in the flow. Turbulent flow static mixers are generally used with fluids that are not very viscous, such as water and gases.
Static mixers are often preferred in many applications as they have no moving parts and therefore require very little maintenance. Energy consumption is also reduced, as no energy is required to drive the mixer, although a pressure drop in the conduit is created by the presence of the mixer therein.
A turbulent flow static mixing device is described in U.S. Pat. No. 4,929,088, which discloses the use of rectangular deflector elements or tabs extending inwardly from the inner wall of a pipe, with the tabs set at an angle to the axis of the pipe such that the tabs extend downstream from the pipe wall. Fluid flows over the upstream faces of the tabs. In practice this system does not work very well because it generates symmetrical vortices in the flow downstream of the tabs. This creates separate vortex zones within the fluid, with little overlap between adjacent zones and little turbulence at the centre of the pipe.
Static mixers are used to mix together one fluid such as chlorine which has been injected into another fluid such as water. If it is desired to inject a small volume of one fluid into another, the use of a mixer which generates separate vortex zones causes problems because the injected fluid tends to stay within the vortex zone into which it was injected, for example either in a vortex zone created by a tab near the pipe wall, or in the less turbulent zone near the axis of the pipe. To overcome this problem, it is necessary to inject fluid into each vortex zone, which is complicated.
A further problem experienced with prior art devices is that if flow rates vary and are periodically low, this often being the case with water systems, then at low flow rates mixing of the injected fluid is inefficient, even with a complex injection pattern. This makes control of the process very difficult. For example, if the rate of injection is controlled by a downstream sensor, monitoring the concentration of the injected agent in the flow, the sensor must be sufficiently far from the injection point for reasonably efficient mixing to have been achieved by the time the fluid passes the sensor. As a result the sensor may be located a long way downstream from the injection point. This makes feedback control systems difficult to stabilise.
A further turbulent static mixer described in U.S. Pat. No. 5,456,533 comprises deflector tabs mounted on a rod which extends across the interior of a pipe. The deflectors are arranged at an angle to the axis of the pipe, with several deflectors being mounted on the rod such that adjacent deflectors are arranged on alternate sides of the rod in a staggered pattern. The tab lengths are either all the same or of very similar length, and adjacent tabs are not separated. The tabs are not arranged in dissimilar sized pairs on opposite sides of a support rod. The mixer creates some turbulence in the flow of fluid in the pipe, but results in a symmetric vortex flow which creates separate vortex zones within the fluid flow, thus leading to inefficient mixing.
It is an object of the present invention to obviate or mitigate some or all of the problems with prior art static mixers as outlined above.
According to the present invention, there is provided a static mixer comprising a group of deflector elements distributed within a conduit through which a fluid may flow in a direction generally parallel to an axis of the conduit, each deflector element defining a surface which is inclined to the conduit axis such that fluid is deflected by the surface in a direction transverse to the axis, wherein the deflector elements are positioned so as to generate an asymmetric vortex flow.
The term asymmetric is used in the sense that there is asymmetry in the vortex flow pattern about the axis of the conduit as the result of using deflector elements which are different in size, shape, or separation, or have different inclination angles with respect to the direction of flow of fluid in the conduits.
Preferably the deflector elements are arranged in pairs of elements having different characteristics, for example rectangular strips of different lengths. The two deflector elements of a pair may extend from a common upstream edge and define between them on a downstream side an included angle of less than 180xc2x0, e.g. 60xc2x0. The two deflector elements of a pair may be equally inclined to the conduit axis, and adjacent pairs of elements may be spaced apart.
Preferably the deflector elements are supported on at least one mounting element extending across the interior of the conduit. Two or more groups of elements may be provided, the mounting elements of the two groups being spaced apart in the direction of the axis and mutually inclined.
Preferably the angle of inclination of at least one of the deflector element surfaces to the conduit axis is adjustable. The angle of inclination may be adjusted in response to fluctuations in flow conditions within the conduit, for example downstream of the deflector elements.
The invention also provides a static mixer comprising a group of deflector elements distributed within a conduit through which a fluid may flow in a direction generally parallel to an axis of the conduit, each deflector element defining a surface which is inclined to the conduit axis such that fluid is deflected by the surface in a direction transverse to the axis, wherein the angle of inclination of at least one of the deflector element surfaces to the conduit axis is adjustable.